1. Field of the Invention
The present invention relates to a powder detecting capacitance-type sensor, and in particular to a level sensor for detecting the height of a powder layer in a tank which holds the powder.
2. Description of the Related Art
FIG. 2. shows an example of an electrostatic powder coating apparatus using a powder detecting capacitance-type sensor. The electrostatic powder coating apparatus 1 charges static electricity to a powder and conducts a uniform and thin coating to an object, which is grounded to the earth, by applying an electric Coulomb force. The powder coating material 2 is supplied from a powder tank 3 to a coating gun 5 through an injector 4. Also the powder coating material 2 is ejected with a carrying air flow from a nozzle opening, which is formed at the end of the coating gun 5, toward the object. At this time, high voltage is charged to electrodes disposed at the end of the coating gun 5 to generate corona discharge from the electrodes of the coating gun 5 toward the object. The powder coating material 2 is charged by ions generated due to the corona discharge and is coated on the surface of the object due to the carrying air flow and electric lines of force.
A capacitance-type sensor 10 is disposed on the side wall 8 of the powder tank 3 and is adapted to sense a powder layer level 7 corresponding to the height of powder coating material 2 in the powder tank 3. In addition to the capacitance-type sensor 10, sensors such a pressure-type, laser-type, sonic-type, vibration-type etc. are used as powder level sensors for detecting the powder layer level 7. However, the capacitance-type sensor 10 has an advantage with respect to the cost and space necessary for accommodating it compared with the other types of sensors. The capacitance-type sensor 10 includes a pair of interior electrodes 11 and detects the existence of powder adjacent the sensor or the amount of powder by utilizing the change of capacitance between the electrodes 11. When the powder coating material 2, of which the dielectric constant is different from that of air, has accumulated in an amount that is high enough to cover the sensor 10, the capacitance between the electrodes 11 changes and the capacitance-type sensor 10 can detect that the powder coating material 2 has accumulated to the determined height.
However, in some cases the powder coating material 2 will attach and deposit on a surface of the capacitance-type sensor 10. In this case, the capacitance-type sensor 10 detects the existence of the powder at all times regardless of the powder layer level 7 in the powder tank 3, and thus there is a problem in that the sensor 10 erroneously detects the powder layer level 7.
The powder coating material 2 can be blown off of the capacitance-type sensor 10 with compressed air 9. However, the effectiveness of the compressed air is not sufficient and the surface of the capacitance-type sensor 10 can become worn due to the powder coating material 2.
Accordingly, an object of the present invention is to provide a capacitance-type sensor for detecting a powder coating material without erroneous detection due to attachment and deposit of the powder coating material on the sensor.
According to an aspect of the present invention, a powder detecting capacitance-type sensor is provided for detecting powder utilizing variance of capacitance between a pair of electrodes. The sensor comprises a sensor body having a pair of interior electrodes. A protection cover covers the sensor body so as to form a compression air chamber between an outer surface of the sensor body and an inner surface of the cover. The protection cover is made of porous material, and an air inlet is communicated with the compression air chamber. Thus, compressed air can be introduced into the compression chamber through the air inlet and blown outside of the protection cover through the protection cover.